Determining characteristics of a fluid in a wellbore

ABSTRACT

An assembly for use in a wellbore can include a plurality of sensors positioned external to a casing string. The plurality of sensors can be positioned for detecting an amount of a hydrocarbon that is present in a fluid in the wellbore and a pH of the fluid in the wellbore. The plurality of sensors can be positioned for wirelessly transmitting signals representing the amount of the hydrocarbon that is present in the fluid and the pH of the fluid to a receiving device.

CROSS-REFERENCES

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 15/535,574, titled “Determining Characteristics ofa Fluid in a Wellbore” and filed Jun. 13, 2017, which claims priority toPCT Application No. US2015/019845, titled “Determining Characteristicsof a Fluid in a Wellbore” and filed Mar. 11, 2015, the entirety of eachof which is hereby incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates generally to devices for use in wellsystems. More specifically, but not by way of limitation, thisdisclosure relates to determining characteristics of a fluid in awellbore.

BACKGROUND

A well system (e.g., an oil or gas well) can include a wellbore that istypically drilled for extracting hydrocarbons from a subterraneanformation. To determine information about the well system (e.g., such asthe commercial viability of the well system), it may be desirable toanalyze fluid in the wellbore. It may be challenging, however, toanalyze fluid in the wellbore.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an example of a well system thatincludes a system for determining characteristics of a fluid in awellbore.

FIG. 2 is a block diagram of an example of a sensor for determiningcharacteristics of a fluid in a wellbore.

FIG. 3 is a block diagram of an example of a computing device fordetermining characteristics of a fluid in a wellbore.

FIG. 4 is a block diagram of an example of a sensor for determiningcharacteristics of a fluid in a wellbore.

FIG. 5 is another cross-sectional view of an example of a well systemthat includes a system for determining characteristics of a fluid in awellbore.

FIG. 6 is a flow chart showing an example of a process for determiningcharacteristics of a fluid in a wellbore.

DETAILED DESCRIPTION

Certain aspects and features of the present disclosure are directed todetermining characteristics of a fluid (e.g., cement) in a wellbore. Insome examples, a sensor is positioned external to a casing string in thewellbore or floating within the fluid in the wellbore. A sensor can bepositioned external to the casing string if it is positioned on orexternal to an outer diameter or outer wall of the casing string. Thesensor can detect an amount of a hydrocarbon in the wellbore, a pH of afluid in the wellbore, an inclination of the wellbore, or anycombination of these well system characteristics. The sensor cantransmit data associated with the well system characteristics (e.g., theamount of the hydrocarbon present in the wellbore) to a receiver via awired or wireless communications interface.

In some examples, the sensor can include a hydrocarbon sensor. Thehydrocarbon sensor can determine the presence of (and an amount of) ahydrocarbon in the wellbore. In some examples, the hydrocarbon sensorcan include two conductive electrodes positioned in parallel and with agap between the two conductive electrodes. The gap can form a fluidcommunication path through which the fluid in the wellbore can flow. Thecombination of the two conductive electrodes with the fluid can form acapacitor in which the fluid is the dielectric material. Upon applyingelectricity across the two conductive electrodes, the sensor can measurean electrical characteristic (e.g., a capacitance, an impedance, or thefrequency of an oscillating waveform) associated with the two conductiveelectrodes. The electrical characteristic can change based on thedielectric properties of the fluid (e.g., the type of the fluid). Thesensor can determine, based on the electrical characteristic, whether ahydrocarbon is present in the fluid (and what type of hydrocarbon ispresent in the fluid).

In some examples, the sensor can include one or more pH sensors. The pHsensors can detect the pH of the fluid in the wellbore. A pH sensor caninclude two different conductive materials, each coupled to avoltage-comparing device (e.g., a comparator, a voltmeter, a computingdevice, or other voltage comparison circuitry). Upon the fluid in thewellbore contacting the two difference conductive materials, a voltagecan be generated across the conductive materials. The voltage-comparingdevice can compare a voltage generated across the two conductivematerials to a reference voltage to determine the pH of the fluid. Inother examples, the pH sensor can include an ionic sensor coupled to avoltage-comparing device. The voltage-comparing device can compare avoltage generated by the ionic sensor to a reference voltage todetermine the pH of the fluid.

These illustrative examples are given to introduce the reader to thegeneral subject matter discussed here and are not intended to limit thescope of the disclosed concepts. The following sections describe variousadditional features and examples with reference to the drawings in whichlike numerals indicate like elements, and directional descriptions areused to describe the illustrative aspects but, like the illustrativeaspects, should not be used to limit the present disclosure.

FIG. 1 is a cross-sectional view of an example of a well system 100 thatincludes a system for determining characteristics of a fluid in awellbore. The well system 100 includes a wellbore 102 extending throughvarious earth strata. The wellbore 102 extends through a hydrocarbonbearing subterranean formation 104. A casing string 106 extends from thesurface 108 to the subterranean formation 104. The casing string 106 canprovide a conduit through which fluid 122, such as production fluidsproduced from the subterranean formation 104, can travel from thewellbore 102 to the surface 108. The casing string 106 can be coupled tothe walls of the wellbore 102. For example, a fluid 105 (e.g., cement)can be pumped between the casing string 106 and the walls of thewellbore 102 for coupling the casing string 106 to the wellbore 102.

The well system 100 can also include at least one well tool 114 (e.g., aformation-testing tool). The well tool 114 can be coupled to a wireline110, slickline, or coiled tube that can be deployed into the wellbore102. The wireline 110, slickline, or coiled tube can be guided into thewellbore 102 using, for example, using a guide 112 or winch. In someexamples, the wireline 110, slickline, or coiled tube can be woundaround a reel 116.

The well system 100 can include one or more sensors 118 a-d. In someexamples, the sensors 118 a-d can include a protective housing (e.g., afluid resistant housing). This can prevent the sensors 118 a-d frombeing damaged by fluid 105, 122, the well tool 114, and debris downhole.

In some examples, a sensor 118 a can include an inclinometer. Theinclinometer can determine the inclination of the well system 100 (e.g.,by detecting the inclination of the casing string 106 to which thesensor 118 a can be coupled). This can be particularly useful if thewell system 100 is an angled well system (e.g., the wellbore 102 isdrilled at an angle between 0 and 90 degrees). Additionally oralternatively, a sensor 118 a can include a pH sensor. The pH sensor candetermine the pH of one or more fluids 105, 122 in the wellbore 102.Examples of pH sensors are further described with respect to FIG. 2. Insome examples, the sensor 118 a can additionally or alternativelyinclude a hydrocarbon sensor. The hydrocarbon sensor can detect thepresence of, or a characteristic of, a hydrocarbon in the wellbore 102.An example of a hydrocarbon sensor is further described with respect toFIG. 4.

In some examples, the sensors 118 a-d can be coupled external to thecasing string 106. This can allow the sensors 118 a-d to monitor thecharacteristics of the well system 100, even if the well tool 114 isremoved or changed. For example, the sensors 118 a-d can be positionedexternal to an outer housing of, or partially embedded within, thecasing string 106. In other examples, the sensors 118 a-d can beconfigured to float within or near the surface of a fluid 105, 122 inthe wellbore 102. For example, as shown in FIG. 1, sensor 118 d isfloating near the within the fluid 105 in the wellbore 102.

In some examples, the sensors 118 a-d can transmit data (e.g., via wiresor wirelessly) associated with the characteristics of the wellbore 102,the fluid 105, 122, or both to a receiver 124 (or to another one of thesensors 118 a-d). The sensors 118 a-d can transmit and receive datausing a transceiver, as described in greater detail with respect to FIG.2. In some examples, the sensors 118 a-d can transmit data using verylow frequency (VLF) magnetic or current pulses, ultrasonic pulses,acoustic pulses, electromagnetic coupling, inductive coupling, or anycombination of these.

One or more receivers 124, 126 can be positioned in the well system 100for receiving data from the sensors 118 a-d. In some examples, thereceivers 124, 126 can be positioned on the well tool 114, on the casingstring 106, or at the surface 108 of the well system 100. The receivers124, 126 can directly or indirectly receive the data from the sensors118 a-d (or other receivers) via a transceiver (which can besubstantially the same as transceiver 216 of FIG. 2). For example, areceiver 124 can wirelessly receive data from a sensor 118 a. Thereceiver 124 can then relay the data via wireline 110 to anotherreceiver 126 at the surface 108. In some examples, the receiver 124 caninclude a distributed acoustic sensor (DAS). A DAS can include afiber-optic device configured to detect acoustic transmissions (e.g.,acoustic emissions) from the sensors 118 a-d. In some examples, thereceiver 124 can use the DAS to receive (e.g., detect) acoustictransmissions from the sensor 118 a-d.

FIG. 2 is a block diagram of an example of a sensor 118 for determiningcharacteristics of a fluid in a wellbore. In some examples, thecomponents shown in FIG. 2 (e.g., the computing device 212, power source214, transceiver 216, pH sensor 200, and pH sensor 201) can beintegrated into a single structure. For example, the components can bewithin a single housing. In other examples, the components shown in FIG.2 can be distributed (e.g., in separate housings) and in electricalcommunication with each other.

The sensor 118 can include a pH sensor 200. The pH sensor 200 caninclude two electrodes 202, 204. The electrodes 202, 204 can includedifferent conductive materials (e.g., different types of metal). Forexample, one electrode 202 can include copper and the other electrode204 can include gold. The electrodes 202, 204 can be exposed forcontacting fluid (e.g., cement) in a wellbore. Upon a fluid contactingthe electrodes 202, 204, a voltage can be generated between theelectrodes 202, 204. The amount of voltage generated between theelectrodes 202, 204 can depend on the pH of the fluid. A comparator 206can compare the voltage generated between the electrodes 202, 204 to areference voltage (e.g., Vref) to determine whether the voltagegenerated between the electrodes 202, 204 is larger or smaller than thereference voltage. In some examples, the reference voltage can becalibrated such that the voltage generated between the electrodes 202,204 is larger than the reference voltage when the fluid contains aparticular pH level or is above a threshold pH level. In some examples,the comparator 206 can transmit a signal associated with the comparisonof the voltages to a computing device 212. The computing device 212 canreceive the signal and determine the pH of the fluid based on thesignal.

The sensor 118 can additionally or alternatively include another pHsensor 201. The other pH sensor can include an ionic sensor 208. Anionic sensor 208 can convert the activity of a specific ion dissolved ina fluid into electrical potential (e.g., voltage). Upon a fluidcontacting the ionic sensor 208, a voltage can be generated that can betransmitted to a comparator 210. The comparator 210 can compare thevoltage generated by the ionic sensor 208 to a reference voltage. Insome examples, the reference voltage can be calibrated such that thevoltage generated by the ionic sensor 208 is larger than the referencevoltage when the fluid contains a particular pH level or is above athreshold pH level. In some examples, the comparator 210 can transmit asignal associated with the comparison of the voltages to a computingdevice 212. The computing device 212 can receive the signal anddetermine the pH of the fluid based on the signal.

In some examples, the sensor 118 can use both pH sensors 200, 201 todetermine the pH of the fluid. For example, the computing device 212 canreceive signals from both pH sensors 200, 201 and compare the signals.If the signals from both pH sensors 200, 201 indicate substantially thesame pH level, the sensor 118 (e.g., the computing device 212) candetermine that the pH measurements are accurate. If the signals from thepH sensors 200, 201 indicate different pH levels, the sensor 118 candetermine that an error occurred. This may provide redundancy, improvingthe accuracy of the sensor 118.

In some examples, the comparators 206, 210 can be replaced withvoltmeters or other voltage measurement circuitry. For example, thecomparator 206 can be replaced with a voltmeter configured to detect thevoltage between the electrodes 202, 204. The voltmeter can transmit asignal associated with the detected voltage to the computing device 212.As another example, the comparator 210 can be replaced with a voltmeterconfigured to detect a voltage generated by the ionic sensor 208 andtransmit a signal associated with the detected voltage to the computingdevice 212. The computing device 212 can receive the signals from thevoltmeters (or other voltage measurement circuitry) and determine a pHof the fluid based on the signals. For example, the computing device 212can use a lookup table to associate a voltage (e.g., from a voltmeter)with a predetermined voltage of a fluid with a particular pH level.

In some examples, the voltage generated by at least one of the pHsensors 200, 201 can be applied to the computing device 212, thetransceiver 216, or both. This voltage can power the computing device212, the transceiver 216, or both. In other examples, components of thesensor 118 (e.g., the computing device 212, the transceiver 216, orboth) can be powered by a power source 214. In some examples, the powersource 214 can include a battery or a wired interface coupled to anelectrical source. The power source 214 can be in electricalcommunication with the computing device 212, and the transceiver 216,and other components of the sensor 118.

In some examples, the computing device 212 can record (e.g., in memory308 of FIG. 3) the pH of the fluid sensed by the sensor 118.Additionally or alternatively, the computing device 212 can transmit(e.g., wired or wirelessly) a signal associated with a fluidcharacteristic (e.g., the pH of a fluid, or an amount of a hydrocarbon),a wellbore characteristic, or both via a transceiver 216.

The sensor 118 can transmit and receive data via the transceiver 216.The transceiver 216 can represent any components that facilitate anetwork connection. In some examples, the transceiver 216 can bewireless and can include wireless interfaces such as IEEE 802.11,Bluetooth, or radio interfaces for accessing cellular telephone networks(e.g., transceiver/antenna for accessing a CDMA, GSM, UMTS, or othermobile communications network). Further, in some examples, thetransceiver 216 can wirelessly transmit data using very low frequency(VLF) magnetic or current pulses, ultrasonic pulses, acoustic pulses,electromagnetic coupling, inductive coupling, or any combination ofthese. In other examples, the transceiver 216 can be wired and caninclude interfaces such as Ethernet, USB, IEEE 1394, or a fiber opticinterface.

FIG. 3 is a block diagram of an example of a computing device 212 fordetermining characteristics of a fluid in a wellbore. The computingdevice 212 can include a processor 304, a memory 308, and a bus 306. Theprocessor 304 can execute one or more operations for operating atransceiver. The processor 304 can execute instructions 310 stored inthe memory 308 to perform the operations. The processor 304 can includeone processing device or multiple processing devices. Non-limitingexamples of the processor 304 include a Field-Programmable Gate Array(“FPGA”), an application-specific integrated circuit (“ASIC”), amicroprocessor, etc.

The processor 304 can be communicatively coupled to the memory 308 viathe bus 306. The non-volatile memory 308 may include any type of memorydevice that retains stored information when powered off. Non-limitingexamples of the memory 308 include electrically erasable andprogrammable read-only memory (“EEPROM”), flash memory, or any othertype of non-volatile memory. In some examples, at least some of thememory 308 can include a medium from which the processor 304 can readthe instructions 310. A computer-readable medium can include electronic,optical, magnetic, or other storage devices capable of providing theprocessor 304 with computer-readable instructions or other program code.Non-limiting examples of a computer-readable medium include (but are notlimited to) magnetic disk(s), memory chip(s), ROM, random-access memory(“RAM”), an ASIC, a configured processor, optical storage, or any othermedium from which a computer processor can read instructions. Theinstructions can include processor-specific instructions generated by acompiler or an interpreter from code written in any suitablecomputer-programming language, including, for example, C, C++, C#, etc.

FIG. 4 is a block diagram of an example of a sensor 118 for determiningcharacteristics of a fluid 408 in a wellbore. In this example, thesensor 118 includes a pH sensor 200, another pH sensor 201, aninclinometer 402, and a hydrocarbon sensor 404.

The hydrocarbon sensor 404 can detect the presence of a hydrocarbon inthe wellbore. In some examples, the hydrocarbon sensor 404 can detectthe presence of a hydrocarbon by measuring a dielectric constant of afluid 408 in the wellbore. Based on the dielectric constant, the sensor118 can determine if the fluid 408 is water, cement, or a hydrocarbon.

For example, the hydrocarbon sensor 404 can include two electrodes 406a, 406 b. The electrodes 406 a, 406 b can include any suitableconductive material, such as gold, silver, copper, or lead. The twoelectrodes 406 a, 406 b can be positioned parallel to each other with agap between the electrodes 406 a, 406 b (e.g., arranged similar tocapacitor plates). The gap can provide a fluid communication paththrough which fluid 408 can flow or otherwise be positioned between theparallel electrodes 406 a, 406 b. The fluid 408 between the electrodes406 a, 406 b can act as a dielectric material. The combination of theparallel electrodes 406 a, 406 b and the fluid 408 acting as adielectric can create a capacitor. In some examples, the sensor 118(e.g., via the computing device 212 or the power source 214) can apply apower signal to the electrodes 406 a, 406 b. The frequency of the powersignal can be within the microwave range of frequencies. This cangenerate an amount of capacitance across the electrodes 406 a, 406 bthat is based on the dielectric constant of the fluid 408. In someexamples, the sensor 118 can measure the capacitance and, based on thecapacitance, can determine the type of the fluid (e.g., whether thefluid is a hydrocarbon or water).

In some examples, a resistor 410 can be positioned in series with thecapacitor (e.g., the electrodes 406 a, 406 b and the fluid 408) in thehydrocarbon sensor 404. This can create a series RC electrical circuit.Upon applying power to the electrodes 406 a, 406 b, the sensor 118 canmeasure a voltage across the resistor (e.g., using a voltmeter coupledacross the resistor). Based on the voltage, the sensor 118 can determinean impedance. Based on the impedance of the capacitor, the sensor 118can determine the type of fluid 408 between the plates.

In still other examples, an inductor (L) can be positioned in serieswith the capacitor (e.g., the electrodes 406 a, 406 b and the fluid408). This can create a series LC electrical circuit. In some examples,the LC circuit can be configured to resonate when the fluid 408 containsa particular hydrocarbon. For example, the inductor can be tuned suchthat the inductive reactance and the capacitive reactance are equal whenthe fluid 408 includes a particular hydrocarbon. In some examples, thesensor 118 can detect whether the LC circuit is resonating and, based onthis detection, determine whether the fluid 408 includes a particularhydrocarbon.

In some examples, the hydrocarbon sensor 404 can include an ionic sensor(not shown). As discussed above, an ionic sensor can convert theactivity of a specific ion dissolved in a fluid into voltage. Becausedifferent fluids can have different levels and types of ions dissolvedin the fluids, different fluids can cause the ionic sensor to generatedifferent voltage levels. In some examples, the sensor 118 candetermine, based on the voltage level from the ionic sensor, the type offluid in the wellbore. For example, the computing device 212 can consulta lookup table (e.g., stored in memory) to determine a fluid in thewellbore using a voltage level from the hydrocarbon sensor 404.

In some examples, the hydrocarbon sensor 404 can use a solid statedevice with a P-N junction and the ionic concentrations of a fluid inthe wellbore to determine a characteristic of the fluid. For example,the hydrocarbon sensor can include an ion-sensitive field-effecttransistor (ISFET). The ISFET can be configured to use the fluid (e.g.,a sample of the fluid) as the gate electrode. Upon an ion concentrationin the fluid changing, current flowing through the ISFET can change. Insome examples, the sensor 118 can measure the current flowing throughthe ISFET and determine, based on a change in the current, the ionicconcentration of the fluid. The sensor 118 can determine, based on theionic concentration, the type of the fluid in the wellbore. In otherexamples, the hydrocarbon sensor 404 can use infrared absorption, or thechemical interaction with a metal junction, to determine ionicconcentration. From the ionic concentration, the sensor 118 candetermine the type of the fluid in the wellbore.

In some examples, the sensor 118 can determine the cumulativeconcentration of a hydrocarbon in a fluid. For example, the sensor 118can determine the cumulative concentration of the hydrocarbon in thefluid by integrating instantaneous concentrations of the hydrocarbonover time: ∫ HydrocarbonConcentration (dt). As another example, thesensor 118 can determine the cumulative concentrations of thehydrocarbon by summing instantaneous concentrations of the hydrocarbonover time: ΣHydrocarbonConcentration*i*Δt, where Δt can represent achange in time.

FIG. 5 is another cross-sectional view of an example of a well systemthat includes a system for determining characteristics of a fluid 514 ina wellbore. In this example, the well system includes a wellbore. Thewellbore can include a casing string 516 and a fluid 518 (e.g., cement)between the casing string 516 and a wall of the wellbore. In someexamples, the wellbore can include a fluid 514 (e.g., mud, cement, or ahydrocarbon) that can flow in an annulus 512 positioned between a welltool 500 and a wall of the casing string 516.

A well tool 500 (e.g., logging-while-drilling tool) can be positioned inthe wellbore. The well tool 500 can include various subsystems 502, 504,506, 507. For example, the well tool 500 can include a subsystem 502that includes a communication subsystem. The well tool 500 can alsoinclude a subsystem 504 that includes a saver subsystem or a rotarysteerable system. A tubular section or an intermediate subsystem 506(e.g., a mud motor or measuring-while-drilling module) can be positionedbetween the other subsystems 502, 504. In some examples, the well tool500 can include a drill bit 510 for drilling the wellbore. The drill bit510 can be coupled to another tubular section or intermediate subsystem507 (e.g., a measuring-while-drilling module or a rotary steerablesystem).

The well system can also include sensors 118 a-d. The sensors 118 a-dcan be positioned external to the casing string 516 (e.g., external toan outer diameter 530 of the casing string 516). For example, thesensors 118 a-d can be positioned on an outer housing 532 of the casingstring 516. As discussed above, the sensors 118 a-d can include aninclinometer, a pH sensor, a hydrocarbon sensor, or any combination ofthese. The sensors 118 a-d can detect characteristics of the fluid 518,514, the wellbore, or both and transmit data associated with thecharacteristics (e.g., to a receiver).

FIG. 6 is a flow chart showing an example of a process for determiningcharacteristics of a fluid in a wellbore according to one example.

In block 602, a sensor positioned in a wellbore detects an amount of ahydrocarbon in the wellbore, a pH of a fluid (e.g., cement) in thewellbore, an inclination of the wellbore, or any combination of these.The sensor can be positioned external to an outer diameter of a casingstring.

In some examples, the sensor can include a hydrocarbon sensor. Thesensor can use the hydrocarbon sensor to determine the amount of thehydrocarbon in the wellbore. In some examples, the hydrocarbon sensorcan include two conductive electrodes positioned in parallel and with agap between the two conductive electrodes. The gap can form a fluidcommunication path through which the fluid in can flow. In someexamples, a sample of the fluid can be positioned in the gap for aperiod of time (e.g., the sensor can take a sample of the fluid andposition the fluid in the gap). Upon applying electricity across the twoconductive electrodes, the sensor can measure an electricalcharacteristic (e.g., a capacitance, impedance, or the frequency of anoscillating waveform) associated with the two conductive electrodes. Thesensor can determine, based on the electrical characteristic, whether ahydrocarbon is present in the fluid (and what type of hydrocarbon ispresent in the fluid).

In some examples, the sensor can include one or more pH sensors. The pHsensors can detect the pH of the fluid in the wellbore. In someexamples, a pH sensor can include two different conductive materialseach coupled to a voltage detection device (e.g., a voltmeter, acomputing device, or other voltage detection circuitry). Using thevoltage detection device, the sensor can detect an amount of voltagegenerated across the two different conductive materials. Based on thedetected amount of voltage, the sensor can determine the pH of thefluid. In other examples, the pH sensor can include an ionic sensorcoupled to a voltage detection device. The sensor can use the voltagedetection device to detect an amount of voltage generated by the ionicsensor. Based on the detected amount of voltage, the sensor candetermine the pH of the fluid.

In some examples, the sensor can include an inclinometer. Theinclinometer can detect the inclination of the wellbore or a well systemcomponent (e.g., a casing string to which the sensor may be coupled).

In block 604, the sensor transmits a signal associated with the amountof the hydrocarbon, the pH of the fluid, the inclination of thewellbore, or any combination of these to a receiving device. The sensorcan transmit the signal wirelessly or via a wired interface. In someexamples, the receiving device can include a DAS sensor. In someexamples, the receiving device can be positioned on or within a welltool in the wellbore, on a cement casing of the wellbore, or at thesurface of the wellbore. In other examples, the receiving device can bepositioned elsewhere in the well system.

In some aspects, systems and methods for determining characteristics ofa fluid in a wellbore are provided according to one or more of thefollowing examples:

Example #1: A system for use in a wellbore can include a plurality ofsensors positioned external to a casing string for detecting an amountof a hydrocarbon that is present in a fluid in the wellbore and a pH ofthe fluid in the wellbore. The plurality of sensors can wirelesslytransmitting signals representing the amount of the hydrocarbon that ispresent in the fluid and the pH of the fluid to a receiving device.

Example #2: The system of Example #1 may feature the plurality ofsensors including a hydrocarbon sensor. The hydrocarbon sensor caninclude two conductive electrodes positioned in parallel and with a gapbetween the two conductive electrodes. The gap can form a fluidcommunication path for the fluid to flow.

Example #3: The system of any of Examples #1-2 may feature a hydrocarbonsensor that includes a processing device and a memory device. The memorydevice can store instructions executable by the processing device forcausing the processing device to: transmit a power signal with afrequency that is within a microwave range of frequencies to twoconductive electrodes; detect an electrical characteristic associatedwith the two conductive electrodes; and determine a type of the fluidbased on the electrical characteristic.

Example #4: The system of any of Examples #1-3 may feature the pluralityof sensors including a pH sensor. The pH sensor can include twodifferent conductive materials each coupled to a voltage-comparingdevice to compare a voltage generated across the two differentconductive materials to a reference voltage to determine the pH of thefluid.

Example #5: The system of any of Examples #1-4 may feature the pluralityof sensors including a pH sensor. The pH sensor can include an ionicsensor coupled to a voltage-comparing device to compare a voltagegenerated by the ionic sensor to a reference voltage to determine the pHof the fluid.

Example #6: The system of any of Examples #1-5 may feature the pluralityof sensors including an inclinometer to detect an inclination of thecasing string and to wirelessly transmit another signal representing theamount of the inclination to the receiving device.

Example #7: A system can include a first sensor that is arranged tofloat in a fluid in a wellbore for detecting an amount of a hydrocarbonthat is present in the fluid and for wirelessly transmitting a signalrepresenting the amount of the hydrocarbon that is present in the fluidto a receiving device. The system can also include a second sensorpositioned external to a casing string for detecting a pH of the fluidand for wirelessly transmitting another signal representing the pH ofthe fluid to the receiving device.

Example #8: The system of Example #7 may feature the first sensorincluding a hydrocarbon sensor. The hydrocarbon sensor can include twoconductive electrodes positioned in parallel and with a gap between thetwo conductive electrodes. The gap can form a fluid communication pathfor the fluid to flow.

Example #9: The system of any of Examples #7-8 may feature a hydrocarbonsensor that includes a processing device and a memory device. The memorydevice can store instructions executable by the processing device forcausing the processing device to: transmit a power signal with afrequency that is within a microwave range of frequencies to twoconductive electrodes; detect an electrical characteristic associatedwith the two conductive electrodes; and determine a type of the fluidbased on the electrical characteristic.

Example #10: The system of any of Examples #7-9 may feature secondsensor including a pH sensor. The pH sensor can include two differentconductive materials each coupled to a voltage-comparing device tocompare a voltage generated across the two different conductivematerials to a reference voltage to determine the pH of the fluid.

Example #11: The system of any of Examples #7-10 may feature the secondsensor including a pH sensor that includes an ionic sensor coupled to avoltage-comparing device to compare a voltage generated by the ionicsensor to a reference voltage to determine the pH of the fluid.

Example #12: The system of any of Examples #7-11 may feature a thirdsensor positioned on an outer housing of the casing string to detect aninclination of the casing string and to wirelessly transmit anothersignal representing the amount of the inclination to the receivingdevice.

Example #13: A system can include a plurality of sensors positioned onan outer housing of a casing string for detecting an amount of ahydrocarbon fluid in a wellbore and an inclination of the casing string,and for wirelessly transmit signals representing the amount of thehydrocarbon fluid and the inclination to a receiving device.

Example #14: The system of Example #13 may feature the plurality ofsensors including a hydrocarbon sensor. The hydrocarbon sensor caninclude two conductive electrodes positioned in parallel and with a gapbetween the two conductive electrodes. The gap can form a fluidcommunication path for a fluid to flow.

Example #15: The system of any of Examples #13-14 may feature ahydrocarbon sensor that includes a processing device and a memorydevice. The memory device can store instructions executable by theprocessing device for causing the processing device to: transmit a powersignal with a frequency that is within a microwave range of frequenciesto two conductive electrodes; detect an electrical characteristicassociated with the two conductive electrodes; and determine a type ofthe fluid based on the electrical characteristic.

Example #16: The system of any of Examples #13-15 may feature theplurality of sensors including a pH sensor. The pH sensor can includetwo different conductive materials each coupled to a voltage-comparingdevice to compare a voltage generated across the two differentconductive materials to a reference voltage to determine the pH of thefluid.

Example #17: The system of any of Examples #13-16 may feature theplurality of sensors including a pH sensor that includes an ionic sensorcoupled to a voltage-comparing device to compare a voltage generated bythe ionic sensor to a reference voltage to determine the pH of thefluid.

Example #18: The system of any of Examples #13-17 may feature each ofthe plurality of sensors including a hydrocarbon sensor and aninclinometer.

Example #19: The system of any of Examples #13-18 may feature each ofthe plurality of sensors including a pH sensor, an inclinometer, and ahydrocarbon sensor.

Example #20: The system of any of Examples #13-19 may feature thereceiving device being positionable in a well tool.

The foregoing description of certain examples, including illustratedexamples, has been presented only for the purpose of illustration anddescription and is not intended to be exhaustive or to limit thedisclosure to the precise forms disclosed. Numerous modifications,adaptations, and uses thereof will be apparent to those skilled in theart without departing from the scope of the disclosure.

What is claimed is:
 1. A system comprising: a first sensor that isconfigured to float in a fluid in a wellbore for detecting an amount ofa hydrocarbon that is present in the fluid and for wirelesslytransmitting a first signal representing the amount of the hydrocarbonthat is present in the fluid to a receiving device; and a second sensorpositionable external to a casing string for detecting a pH of the fluidand for wirelessly transmitting a second signal representing the pH ofthe fluid to the receiving device; wherein the second sensor includes acomputing device and a transceiver; and wherein the second sensor isconfigured to supply a voltage to a power port of the computing deviceor the transceiver to power the computing device or the transceiver,respectively.
 2. The system of claim 1, wherein the first sensorcomprises a hydrocarbon sensor that includes two conductive electrodespositioned in parallel and with a gap between the two conductiveelectrodes, wherein the gap forms a fluid communication path for thefluid to flow.
 3. The system of claim 2, wherein the hydrocarbon sensorcomprises: a processing device; and a memory device in whichinstructions executable by the processing device are stored for causingthe processing device to: transmit a power signal with a frequency thatis within a microwave range of frequencies to the two conductiveelectrodes; detect an electrical characteristic associated with the twoconductive electrodes; and determine a type of the fluid based on theelectrical characteristic.
 4. The system of claim 1, wherein the voltageis a first voltage, and wherein the second sensor includes two differentconductive materials coupled to a voltage-comparing device, thevoltage-comparing device being configured to compare a second voltagegenerated across the two different conductive materials to a referencevoltage for determining the pH of the fluid.
 5. The system of claim 4,wherein the voltage-comparing device includes a comparator.
 6. Thesystem of claim 1, wherein the second sensor includes an ionic sensor.7. The system of claim 1, further comprising a third sensor positionedon a the casing string, the third sensor being configured to: detect aninclination of the casing string; and wirelessly transmit a third signalrepresenting the inclination to the receiving device.
 8. The system ofclaim 1, wherein the second sensor is a first pH sensor, the systemfurther includes a second pH sensor, and the computing device isconfigured to: receive a first sensor signal from the first pH sensorand a second sensor signal from the second pH sensor; determine that afirst pH level represented by the first sensor signal is substantiallysimilar to a second pH level represented by the second sensor signal;and in response to determining that the first pH level is substantiallysimilar to the second pH level, determining that the first pH level isaccurate.
 9. The system of claim 1, wherein the first sensor includes aninductor that is electrically coupled in series to a capacitor to forman inductor-capacitor (LC) circuit, the LC circuit being tuned toresonate when the fluid is a particular type of hydrocarbon.
 10. Thesystem of claim 1, wherein the second sensor is configured to power thecomputing device or the transceiver as an alternative to a power source.11. A system comprising: a first sensor that is configured to float in afluid in a wellbore for detecting an amount of a hydrocarbon that ispresent in the fluid and for wirelessly transmitting a first signalrepresenting the amount of the hydrocarbon that is present in the fluidto a receiving device; a second sensor that is positionable external toa casing string of the wellbore for detecting a characteristic of thefluid in the wellbore and for wirelessly transmitting a second signalrepresenting the characteristic of the fluid to the receiving device; acomputing device; and a transceiver; wherein the second sensor isconfigured to supply a voltage to a power port of the computing deviceor the transceiver to power the computing device or the transceiver,respectively.
 12. The system of claim 11, wherein the characteristic isa pH of the fluid.
 13. The system of claim 11, wherein the second sensorincludes the computing device and the transceiver.
 14. The system ofclaim 11, wherein the voltage is a first voltage and the second sensorincludes two different conductive materials coupled to avoltage-comparing device, the voltage-comparing device being configuredto compare a second voltage generated across the two differentconductive materials to a reference voltage.
 15. The system of claim 14,wherein the voltage-comparing device includes a comparator.
 16. Thesystem of claim 11, wherein the second sensor is positioned on thecasing string, the second sensor being configured to: detect aninclination of the casing string; and wirelessly transmit a signalrepresenting the inclination to the receiving device.
 17. The system ofclaim 11, wherein the second sensor is a first PH sensor, the systemfurther includes a second pH sensor, and the computing device isconfigured to: receive a first sensor signal from the first pH sensorand a second sensor signal from the second pH sensor; determine that afirst pH level represented by the first sensor signal is substantiallysimilar to a second pH level represented by the second sensor signal;and in response to determining that the first PH level is substantiallysimilar to the second pH level, determining that the first pH level isaccurate.
 18. The system of claim 11, wherein the second sensor isconfigured to power the computing device or the transceiver as analternative to a power source.
 19. A method comprising: detecting, by afirst sensor floating in a fluid in a wellbore, an amount of ahydrocarbon that is present in the fluid; wirelessly transmitting, bythe first sensor, a first signal representing the amount of thehydrocarbon that is present in the fluid to a receiving device;detecting, by a second sensor positioned external to a casing string ofthe wellbore, the a characteristic of the fluid; wirelesslytransmitting, by the second sensor, a second signal representing thecharacteristic of the fluid to the receiving device; generating, by thesecond sensor, a voltage; and supplying, by the second sensor, thevoltage to a power port of a computing device or a transceiver to powerthe computing device or the transceiver, respectively.